1. Field of the Invention
The present invention relates to improved composite materials and the infiltration methods for producing the same. Specifically, the present invention relates to complex-shaped composite bodies produced from a bonded assemblage of smaller, simpler shapes. Even more specifically, the present invention relates to silicon carbide composites wherein preferably at least a portion of the silicon carbide is produced by reactive infiltration.
2. Discussion of Related Art
Silicon carbide composites have been produced by reactive infiltration techniques for more than thirty-five years. In general, such a reactive infiltration process comprises contacting molten silicon with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment. A wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide. This in-situ silicon carbide typically is interconnected. A dense body usually is desired, so the process typically occurs in the presence of excess silicon. The resulting composite body thus comprises silicon carbide and unreacted silicon (which typically also is interconnected), and may be referred to in shorthand notation as Si/SiC or RBSC (denoting “reaction-bonded silicon carbide”).
In one of the earliest demonstrations of this technology, Popper (U.S. Pat. No. 3,275,722) produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300 C.
Taylor (U.S. Pat. No. 3,205,043) also produced dense silicon carbide bodies by reactively infiltrating silicon into a porous body containing silicon carbide and free carbon. Unlike Popper, Taylor first made a preform consisting essentially of granular silicon carbide, and then he introduced a controlled amount of carbon into the shaped mass. In one embodiment of his invention, Taylor added the carbon in the form of a carbonizable resin, and then heated the mass containing the silicon carbide and infiltrated resin to decompose (carbonize) the resin. The shaped mass was then heated to a temperature of at least 2000 C in the presence of silicon to cause the silicon to enter the pores of the shaped mass and react with the introduced carbon to form silicon carbide.
Hillig and his colleagues at the General Electric Company took a different approach, where fibrous versions of Si/SiC composites were produced by reactively infiltrating carbon fiber preforms.
More recently, Chiang et al. (U.S. Pat. No. 5,509,555) discloses the production of silicon carbide composite bodies through the use of a silicon alloy infiltrant. The preform to be infiltrated by the alloy can consist of carbon or can consist essentially of carbon combined with at least one other material such as a metal like Mo, W, or Nb; a carbide like SiC, TiC, or ZrC; a nitride like Si3N4, TiN or AlN; an oxide like ZrO2 or Al2O3; or an intermetallic compound like MoSi2 or WSi2, or mixtures thereof The liquid infiltrant includes silicon and a metal such as aluminum, copper, zinc, nickel, cobalt, iron, manganese, chromium, titanium, silver, gold, platinum and mixtures thereof.
In a preferred embodiment of the Chiang et al. invention, the preform can be a porous carbon preform, the liquid infiltrant alloy can be a silicon-aluminum alloy containing in the range of from about 90 at % to about 40 at % silicon and in the range of from about 10 at % to about 60 at % aluminum and the carbon preform can be contacted with the silicon-aluminum alloy at a temperature in the range of from about 900 C to about 1800 C for a time sufficient so that at least some of the porous carbon is reacted to form silicon carbide. Upon cooling, the dense composite formed thereby can be characterized by a phase assemblage comprising silicon carbide and at least one phase such as silicon-aluminum alloy, a mixture of silicon and aluminum, substantially pure aluminum or mixtures thereof.
M. Singh describes the joining of silicon carbide-based ceramic materials for high temperature applications. Specifically, he applies a carbonaceous mixture to the join area, and then cures the carbonaceous material at 110-120 C for 10 to 20 minutes to fasten the pieces together. Then silicon or silicon alloy is applied to the joint region and heated to 1250-1425 C for 5 to 10 minutes. The molten silicon or silicon alloy reacts with the carbon to form silicon carbide with controllable amounts of silicon and other phases as determined by the alloy composition. This reaction forming approach has been used to produce strong joints in commercially available reaction-bonded and sintered silicon carbide-based materials. Singh states that his reaction forming technique is unique in its ability to produce joints with tailorable microstructures, but he advances no suggestion of incorporating one or more filler materials into the joint region. He does recognize, however, that it may be important to tailor the thermomechanical properties of the joint region to be close to those of the silicon carbide-based materials being joined. (M. Singh, Industrial Heating, Sep. 1997, pp. 91-93)
U.S. Pat. No. 4,070,197 to Coes discloses the formation of a gas impermeable hollow silicon carbide body. Specifically, Coes first forms two separate hollow silicon carbide bodies by a slip-casting method. Then the two separate bodies are cemented together by means of a silicon carbide slip, preferably containing a binder such as sodium silicate. The joined pieces are then fired at a temperature sufficiently high as to recrystallize the silicon carbide, thereby forming a recrystallized joint between the pieces. The body is then exposed to a silicon atmosphere, which deposits silicon in the body to form a dense, gas-impermeable structure. In a modification of the preferred embodiment, additional carbon may be provided in the product prior to the final siliconizing operation so as to form additional grains of silicon carbide in the final fired and siliconized structure.
It is an object of the present invention to produce a silicon carbide composite body to near-net shape, thereby minimizing the amount of grinding and/or machining necessary to achieve the required dimensions of the finished article.
It is an object of the present invention to produce a solid, strong, unitary-silicon carbide composite structure from a bonded assemblage of smaller structures.
It is an object of the present invention to be able to produce a solid, strong, unitary silicon carbide composite structure of a shape that might otherwise be too complex or too difficult to produce as a single body from its inception.
It is an object of the present invention to provide a more reliable method to temporarily bond preforms to one another until subsequent infiltration provides a permanent bond.
It is an object of the present invention to provide at least one metal to the silicon infiltrant to produce a silicon carbide composite body containing some of this metal, thereby enhancing its properties and further extending its capabilities.